Electrometers are instruments for measuring electric charge and thus, can be configured to measure either small voltages or small currents. The present invention is an instrument for measuring small currents.
The measurement of a small electrical current in the range of 1.0 E-15 amps is usually accomplished through the use of an electrometer amplifier configured as a current to voltage converter. This configuration makes use of a high megohm feedback resistor, of the order of 1. 0 E+12 ohms, to develop a useful voltage from the small currents being measured. However, the resistor also acts as a noise source and generates a noise voltage defined by either the Johnson or the thermal noise equation. The thermal noise equation can be expressed as follows: EQU E=SQRT[(4K)(T)(f)(R)]
where SQRT=square root
E=volts RMS noise
K=Boltzman's constant
T=Temperature in degrees Celsius
f=noise bandwidth for the measurement in hertz
R=resistance in ohms
The subject invention eliminates the high megohm resistor and its associated noise, E, by using a small, typically 10 to 20 picofarad, capacitor as the sole feedback element in the electrometer configuration. The electrometer output voltage using this latter configuration is proportional to the input current multiplied by the time the input current is applied divided by the value of the feedback capacitance. An equation depicting this relationship can be expressed as follows: EQU E=(I)(t)/C
where E=output voltage of the electrometer in volts
I=input current in amps
t=length of time input current is applied in seconds
C=feedback capacitor value in farads
Thus, for this configuration, the electrometer output voltage increases linearly for a constant input current; however, when employing this system, the capacitor must be reset or zeroed before the electrometer amplifier reaches its saturation voltage. To accomplish this, the subject invention automatically resets or zeros the accumulated charge on the feedback capacitor when the voltage level of the electrometer amplifier approaches saturation.
The subject invention employs a data collection system which assimilates voltage data over fixed segments of time and then uses this data to determine the current from the above cited relationship E=(I)(t)/(C).
Accordingly, it is an object of this invention to provide an electrometer which is capable of measuring small currents without experiencing the noise generated by a feedback resistor.
It is still a further object of this invention to provide a means to automatically reset or zero the accumulated charge on the feedback capacitor as required.
A further object of this invention is to provide a means to automatically compile the data from the electrometer and to determine the input current based on the accumulated data.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.